# Tachometer

I am interested in purchasing a tachometer for my team’s use. Can anyone recommend a good quality one? Or at least give me some feedback on some they have used? Thanks a lot.

I’m not quite sure exactly what you are trying to do, but many teams measure motor speed using some sort of wheel with light and dark spots, or opaque and transparent spots, and use light sensors (such as the Banner sensors in the kit).

I am interested in purchasing a tachometer for my team’s use. Can anyone recommend a good quality one? Or at least give me some feedback on some they have used? Thanks a lot.

Theoretically it is possible to use the current sensors as a tachometer. Since current is releated to torque and torque is releated to speed. All you would need to do is solve for some fairly simple math.

Again, it depends on the intended use. There are several kinds of tachs that one can use depending on what you plan to measure and how you plan to measure it.

Generally speaking though, contact type dial tachs can be had for less than \$100. Phototachs (contact and non-contact) range from around \$100 to \$300, while laser tachs start around \$200 up to \$300. These are all the hand hand type.

Unless you have a specific need for a contact type, then one of the contactless tachometers should be the one of choice.

Not quite. Current is related to voltage which is related to rotational velocity. Close though.

Matt

Not quite. Current is related to voltage which is related to rotational velocity. Close though.

Im pretty sure that torque is a function of current and some constant, and rpm’s are a function of voltage and some constant. Of course this is only for an ideal motor. These equations are related to each other though.

the current draw method would only work until your bot hit something like a wall. The draw would get really high and throw off the rest of the program excecution.

You can buy low cost fairly reliable tach’s from Tower Hobbies, they are non contact, usually used for sensing the speed of a prop on an RC plane.

Zach,
McMaster-Carr has several in their catalog. (2-3 pages) A non-contact one would serve you best but these are not cheap devices. \$150-\$400. Although there was a pocket one that was at the low end of the scale. Unless you are using it for analysis or experimentation it is not a really useful device for our applications. You can only use it when the robot is off the floor so the speed you are measuring is the unloaded speed. When the robot is running you won’t be able to follow it around and get a reading especially if the shafts are buried in the body of the robot.
If you are using it for experimentation or demonstration then the cheapest one available will show enough info to be useful.

As has been posted, you can use current draw, but I wouldn’t use the sensors that came with the bot. They are extremely inaccurate and fluctuate constantly. If you really do want to take this approach, I would suggest looking for a third party supplier for a decent current sensor.

[EDIT] You could also use a condenser(I think thats what the potentiometers that can spin all the way around over and over all called) and just poll in your programming whenever they loop back to zero.

Guys,
There are too many variables involved in using current to determine RPM. Remember that there are frictional changes, weight changes (if the robot picks up stuff) load changes when pushing or pulling and the possibility that the monitored wheel will not be in contact with the floor. Add to that, changing resistance in components as they change temperature, and internal resistance in the battery all of which affect the current delivered to a motor. With that said, those teams that have used current sensing in the past have a much better understanding of motor interaction and robot function once they look at current draw vs. match play and monitoring battery terminal voltage at the same time is also very helpful.
Anthony, a condensor is another name for capacitor. It goes back into the previous century. A pot that has continuous rotation is simply know as a continuous pot, a no limit pot or a 360 degree pot. What you may be thinking of is a shaft encoder which will output a variety of signals depending on design.
We used two banner sensors aimed at the wheel on one corner of our robot. Any two sensors (separated by real world conditions) triggered by the same stimulus will output a pulse that is time different from the other. By observing (in software or hardware) which pulse occurs before the other you can determine not only RPM but direction. Depending on how many triggers you get in one rotation of the wheel will determine your accuracy. Ours is accurate down to about one inch of travel.

Harbor Freight has a non contact photo tachometer. It cost \$69.99. I bought one a couple of years back. I used it to confirm the freeload speed of the motors with and without our transmission. Easy to use. Your supposed to use some reflective tape and just point the tach at it. But you can also use black tape or white tape just as long as you make sure only one portion of the area on the shaft/wheel your measuring is reflective. It was interesting to confirm that drill motors do indeed spin slower in reverse.

Your school may already have a measuring device. See if the Physics teacher has a strobescope. Its a little harder to use, but you just mark a spot on the shaft or make a small flag out of tape and turn it on. Just vary the frequency of the strobe so that your mark or tape flag looks stationary and read off the frequency which will also be the rpm. Nice thing about this is that its also non contact. Just watch out for harmonics as they can give you a false reading.